They used to be covered by `optin_builtin_traits` but negative impls
are now applicable to all traits, not just auto traits.

This also adds docs in the unstable book for the current state of auto traits.

Rollup of 6 pull requests

Successful merges:

 - #69866 (Rename `def_span` to `guess_head_span`)
 - #69878 (Tweak chained operators diagnostic)
 - #70375 (avoid catching InterpError)
 - #70386 (typeck: minor pattern typing improvements)
 - #70389 (borrowck: prefer "value" over "`_`" in diagnostics)
 - #70395 (Update cargo.)

Failed merges:

r? @ghost

Update cargo.

8 commits in 7019b3ed3d539db7429d10a343b69be8c426b576..8a0d4d9c9abc74fd670353094387d62028b40ae9
2020-03-17 21:02:00 +0000 to 2020-03-24 17:57:04 +0000
- Re-implement proc-macro feature decoupling. (rust-lang/cargo#8028)
- Remove unused transitive dependencies: miniz_oxide, adler32 (rust-lang/cargo#8023)
- Fix bug with -Zfeatures=dev_dep and `check --profile=test`. (rust-lang/cargo#8027)
- Remove Config from CompileOptions. (rust-lang/cargo#8021)
- Add `rustless.org` to documented blocklist. (rust-lang/cargo#7922)
- Print colored warnings when build script panics (rust-lang/cargo#8017)
- Do not supply --crate-version flag to rustdoc if present in RUSTDOCFLAGS (rust-lang/cargo#8014)
- Add proc-macro to index, and new feature resolver. (rust-lang/cargo#8003)

borrowck: prefer "value" over "`_`" in diagnostics

Fixes https://github.com/rust-lang/rust/issues/67565.

r? @pnkfelix @matthewjasper
cc @mark-i-m

typeck: minor pattern typing improvements

r? @estebank

avoid catching InterpError

Avoid raising and then capturing `InterpError` for the definedness check.

Cc https://github.com/rust-lang/rust/issues/69297
r? @oli-obk

Tweak chained operators diagnostic

Use more selective spans
Improve suggestion output
Be more selective when displaying suggestions
Silence some knock-down type errors

r? @Centril

Rename `def_span` to `guess_head_span`

r? @EddyB

Use more selective spans
Improve suggestion output
Be more selective when displaying suggestions
Silence some knock-down type errors

Rollup of 5 pull requests

Successful merges:

 - #69700 (Rename LayoutDetails to just Layout.)
 - #70392 (Make x.py compatible with python 3.8.)
 - #70406 (Clean up E0458 explanation)
 - #70407 (Avoid tagging as I-nominated on toolstate breakage)
 - #70409 (gitignore: allow target to be a symlink)

Failed merges:

 - #70375 (avoid catching InterpError)

r? @ghost

gitignore: allow target to be a symlink

Following rust-lang/cargo#4944.

Avoid tagging as I-nominated on toolstate breakage

r? @Mark-Simulacrum

Clean up E0458 explanation

r? @Dylan-DPC

Make x.py compatible with python 3.8.

Python 3.8 removes the `time.clock()` function, use `time.perf_counter()` instead.

Rename LayoutDetails to just Layout.

Following rust-lang/cargo#4944.

This reverts commit 46a8fcdf.

Rollup of 5 pull requests

Successful merges:

 - #70226 (use checked casts and arithmetic in Miri engine)
 - #70319 (correctly normalize constants)
 - #70352 (Add long error explanation for E0710 )
 - #70366 (Implement Fuse with Option)
 - #70379 (fix incorrect type name in doc comments)

Failed merges:

 - #70375 (avoid catching InterpError)

r? @ghost

fix incorrect type name in doc comments

Change : `InterpCtx` => `InterpCx`
(`rustc_mir::interpret::InterpCx`)

Implement Fuse with Option

The former `done` flag was roughly similar to an `Option` tag, but left
the possibity of misuse. By using a real `Option`, we can set `None`
when the iterator is exhausted, removing any way to call it again. We
also allow niche layout this way, so the `Fuse` may be smaller.

The `FusedIterator` specialization does want to ignore the possibility
of exhaustion though, so it uses `unsafe { intrinsics::unreachable() }`
to optimize that branch away. The entire `Fuse` implementation is now
isolated in its own module to contain that unsafety.

r? @scottmcm

Add long error explanation for E0710

Add long explanation for the E0710 error code
Part of #61137

r? @GuillaumeGomez

correctly normalize constants

closes #70317

implements https://github.com/rust-lang/rust/issues/70125#issuecomment-602133708

r? eddyb cc @varkor

use checked casts and arithmetic in Miri engine

This is unfortunately pretty annoying because we have to cast back and forth between `u64` and `usize` more often that should be necessary, and that cast is considered fallible.

For example, should [this](https://doc.rust-lang.org/nightly/nightly-rustc/rustc/mir/interpret/value/enum.ConstValue.html) really be `usize`?

Also, `LayoutDetails` uses `usize` for field indices, but in Miri we use `u64` to be able to also handle array indexing. Maybe methods like `mplace_field` should be suitably generalized to accept both `u64` and `usize`?

r? @oli-obk Cc @EddyB

Python 3.8 removes the time.clock() function, use time.perf_counter() instead.

add helper method for ptr ops on Scalar; reduce unnecessary large operand of overflowing_signed_offset